Percussion enhanced rotary drilling is known per se. A percussion enhanced rotary drill comprises a rotary drill bit and an oscillator for applying oscillatory loading to the rotary drill bit. The oscillator provides impact forces on the material being drilled so as to break up the material which aids the rotary drill bit in cutting though the material.
Resonance enhanced rotary drilling is a special type of percussion enhanced rotary drilling in which the oscillator is vibrated at high frequency so as to achieve resonance with the material being drilled. This results in an amplification of the pressure exerted at the rotary drill bit thus increasing drilling efficiency when compared to standard percussion enhanced rotary drilling.
U.S. Pat. No. 3,990,522 discloses a percussion enhanced rotary drill which uses a hydraulic hammer mounted in a rotary drill for drilling bolt holes. It is disclosed that an impacting cycle of variable stroke and frequency can be applied and adjusted to the natural frequency of the material being drilled to produce an amplification of the pressure exerted at the tip of the drill bit. A servovalve maintains percussion control, and in turn, is controlled by an operator through an electronic control module connected to the servovalve by an electric conductor. The operator can selectively vary the percussion frequency from 0 to 2500 cycles per minute (i.e. 0 to 42 Hz) and selectively vary the stroke of the drill bit from 0 to ⅛ inch (i.e. 0 to 3.175 mm) by controlling the flow of pressurized fluid to and from an actuator. It is described that by selecting a percussion stroke having a frequency that is equal to the natural or resonant frequency of the rock strata being drilled, the energy stored in the rock strata by the percussion forces will result in amplification of the pressure exerted at the tip of the drill bit such that the solid material will collapse and dislodge and permit drill rates in the range 3 to 4 feet per minute.
There are several problems which have been identified with the aforementioned arrangement and which are discussed below.
High frequencies are not attainable using the apparatus of U.S. Pat. No. 3,990,522 which uses a relatively low frequency hydraulic oscillator. Accordingly, although U.S. Pat. No. 3,990,522 discusses the possibility of resonance, it would appear that the low frequencies attainable by its oscillator are insufficient to achieve resonance enhanced drilling through many hard materials.
Regardless of the frequency issue discussed above, resonance cannot easily be achieved and maintained in any case using the arrangement of U.S. Pat. No. 3,990,522, particularly if the drill passes through different materials having different resonance characteristics. This is because control of the percussive frequency and stroke in the arrangement of U.S. Pat. No. 3,990,522 is achieved manually by an operator. As such, it is difficult to control the apparatus to continuously adjust the frequency and stroke of percussion forces to maintain resonance as the drill passes through materials of differing type. This may not be such a major problem for drilling shallow bolt holes as described in U.S. Pat. No. 3,990,522. An operator can merely select a suitable frequency and stroke for the material in which a bolt hole is to be drilled and then operate the drill. However, the problem is exacerbated for deep-drilling through many different layers of rock. An operator located above a deep-drilled hole cannot see what type of rock is being drilled through and cannot readily achieve and maintain resonance as the drill passes from one rock type to another, particularly in regions where the rock type changes frequently.
Some of the aforementioned problems have been solved by the present inventor as described in WO 2007/141550. WO 2007/141550 describes a resonance enhanced rotary drill comprising an automated feedback and control mechanism which can continuously adjust the frequency and stroke of percussion forces to maintain resonance as a drill passes through rocks of differing type. The drill is provided with an adjustment means which is responsive to conditions of the material through which the drill is passing and a control means in a downhole location which includes sensors for taking downhole measurements of material characteristics whereby the apparatus is operable downhole under closed loop real-time control.
Despite the solutions described in WO 2007/141550, there is a desire to make further improvements to the methods and apparatus described therein. It is an aim of embodiments of the present invention to make such improvements in order to increase drilling efficiency while limiting wear and tear on the apparatus so as to increase the lifetime of the apparatus. It is a further aim to more precisely control resonance enhanced drilling, particularly when drilling through rapidly changing rock types.